Problem: Solve for $r$ : $11 + r = -9$
Answer: Subtract $11$ from both sides: $ \begin{eqnarray} \\ 11 + r &=& -9 \\ \\ {-11} && {-11} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{11 + r} &\hphantom{=}& \hphantom{-9} \\ r &=& -9 {- 11} \\ \end{eqnarray} $ Simplify. $ r = -20$